Preseed side effects?

So I used preseed two times, both days that I ovulated last month. My Coke lasts from 30-37 days. 37 days only happened one time and that has been my longest cycle. I tested yesterday(38 days) and it was negative. Today is 40 days. The only thing I've done differently is use preseed. I've had cramps, that feel like Aunt Flo is coming since day 35. But it's on and off. Could preseed be making me late? I know that might be a silly question. But I don't understand why I'm so late and with a negative test.